Power circuit for battery

ABSTRACT

A power circuit for a battery for, even when an idle-stop operation is continuously performed, preventing reduction of an electric power supplied to a motor at start-up to obtain a set engine rpm. The power circuit includes a series-connected power supply in which a battery having a load and a capacitor group are connected in series with each other, a DC/DC converter for shifting electric power between the battery and the capacitor group, and between the battery and the load, and a controller for controlling the DC/DC converter. The controller detects the voltage of the capacitor group, and when the voltage detected is lower than a first threshold voltage, controls the DC/DC converter so that the capacitor group is charged with electricity.

TECHNICAL FIELD

The present invention relates to a power circuit for a battery, and moreparticularly to a power circuit for a battery which is installed in avehicle such as an automobile to be used therein.

BACKGROUND ART

In the conventional power circuit for a battery, there is a problem inthat if an idle-stop operation (stop/start-up operation) is continuouslycarried out, then the recharging for a capacitor group connected inseries with a battery group becomes insufficient, so that it becomesimpossible to supply a sufficient electric power to a motor through aninverter, and hence a predetermined start-up operation by the motor of avehicle cannot be carried out. A predetermined start-up operation meansan operation for increasing the rpm of an engine from a stop state to anidle running state (engine rpm of about 800) with the motor.

In the conventional power circuit for a battery, there is a problem inthat if an idle-stop operation (stop/start-up operation) is continuouslycarried out, then the recharging for a capacitor group connected inseries with a battery group becomes insufficient so that it becomesimpossible to supply a sufficient electric power to a motor through aninverter, and hence a predetermined start-up operation by the motor of avehicle can not be carried out. The predetermined start-up operationmeans an operation for increasing the rpm of an engine from a stop stateto an idle running state (engine rmp of about 800) by the motor.

In addition, there arises a problem in that a sufficient motor outputcan not be obtained due to insufficiency in voltage of the capacitor,and hence the start-up by the motor can not be carried out.

In addition, if the idle-stop operation is continuously carried out,then a sufficient period of time cannot be secured to recharge thecapacitor with electricity. Thus, the capacitor voltage at the start-upoperation takes various values, and the start-up operation is carriedout in this state. Then, there arises a problem in that if the DC/DCconverter is operated with a fixed output independently of the capacitorvoltage, then the efficiency of the overall battery drive circuit systemincluding a battery, a capacitor, and a DC/DC converter deterioratesdepending on the capacitor voltage values. The deterioration of theefficiency causes an increase in a quantity of calorification of theoverall system. In particular, there is a concern about reduced life ofthe capacitor due to rise in temperature caused by the calorification,and the heating of other apparatuses.

In addition, due to internal resistances that exist in the capacitor andthe battery, there is a possibility that in outputting a large electricpower at the engine restart-up or the like, a voltage drop due to theinternal resistance exerts adverse influences on other on-vehicleapparatuses. In particular, when a charge voltage of the capacitor islow or a state of charge (SOC) of the battery is low, there is a problemin that a battery current required for restart-up of an engine isincreased to decrease a battery output voltage.

In regeneration of energy, a series-connected body of the battery andthe capacitor is charged with the energy generated by the electricmotor. The permissible output electric power density of a lead acidbattery is low, about 100 to 200 W/kg, and allowable input electricpower density is even lower. For this reason, charge current duringregeneration is determined on the basis of the allowable input currentof the battery. Note that the allowable input electric power density ofthe battery is proportional to the allowable input current since thevoltage of the battery is nearly fixed. Thus, the high speed chargingcharacteristics of the capacitor can not be utilized, and hence it isnecessary to limit the regeneration output of the electric motor.

DISCLOSURE OF THE INVENTION

The present invention is made to solve the above-mentioned problems, andit is an object of the present invention to provide a power circuit fora battery which is capable of, even when an idle-stop operation iscontinuously performed, preventing reduction of an electric powersupplied to a motor at start-up to obtain a predetermined engine rpm.

In addition, it is a second object of the present invention to obtain apower circuit for a battery which is capable of regenerating aninstantaneous large energy generated by an electric motor during, forexample, breaking on a vehicle, without damaging a battery.

A power circuit for a battery according to the present inventionincludes: a first energy storage source; a second energy storage sourceconnected in series with the first energy storage source; a DC/DCconverter for shifting an electric power between the first energystorage source and the second energy storage source; and control meansfor controlling the DC/DC converter.

The control means of the present invention detects a voltage of theenergy storage source that is disposed on a high voltage side, fromamong the first and second energy storage sources of theseries-connected power supply. When the detected voltage is lower than apredetermined first threshold voltage, the energy storage sourcedisposed on the high voltage side is charged with electricity by theDC/DC converter. Thus, even when the idle-stop operation is continuouslycarried out, it is possible to prevent reduction of the electric powersupplied to the motor at start-up, and hence it is possible to obtainthe predetermined engine rpm.

In addition, the power circuit for a battery of the present inventionfurther includes an electric power conversion circuit for shifting anelectric power between an electric motor connected to an axle of avehicle, and the first and second energy storage sources connected inseries with each other. When the first and second energy storage sourcesare charged with the regenerative electric power of the electric motorthrough the electric power conversion circuit, the control meanscontrols the DC/DC converter so that an input current to the firstenergy storage source becomes equal to or smaller than an allowableinput current of the first energy storage source. Hence, it is possibleto increase a charge electric power for a battery and a capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a configuration of a power circuitfor a battery according to the present invention;

FIG. 2 is an explanatory diagram showing a calculation model fordetermining a control condition for a DC/DC converter provided in thepower circuit for a battery according to the present invention;

FIG. 3 is an explanatory diagram showing a relationship between a chargevoltage and an output voltage of a capacitor group for each output of aDC/DC converter provided in the power circuit for a battery according tothe present invention;

FIG. 4 is an explanatory diagram showing a relationship between acapacitor initial voltage value and the number of times of idle-start(continuous operation) in a conventional power circuit for a battery;

FIG. 5 is a flow chart showing a control method in the power circuit fora battery according to the present invention;

FIG. 6 is an explanatory diagram showing a relationship between abattery voltage and a threshold voltage of a capacitor with which therpm of a motor can be increased to the desired rpm in the power circuitfor a battery according to the present invention;

FIG. 7 is an explanatory diagram showing a relationship between acapacitor voltage for each output electric power of a DC/DC converterand a system efficiency when a battery, a capacitor group, and the DC/DCconverter are regarded as one power circuit in the power circuit for abattery according to the present invention;

FIG. 8 is an explanatory diagram showing a relationship between acapacitor voltage and an output threshold voltage of a DC/DC converterin the power circuit for a battery according to the present invention;

FIG. 9 is a flow chart showing an operation of a power circuit for abattery according to Embodiment 2 of the present invention;

FIG. 10 is an explanatory diagram showing a relationship between a DC/DCconverter output and a battery terminal voltage in the power circuit fora battery according to Embodiment 2 of the present invention;

FIG. 11 is a circuit diagram showing a configuration of a power circuitfor a battery according to Embodiment 3 of the present invention;

FIG. 12 is a detailed block diagram of a control circuit shown in FIG.11;

FIG. 13 is a flow chart of regenerative control shown in FIG. 11;

FIG. 14 is a flow chart of regenerative control shown in FIG. 11;

FIG. 15 is a diagram showing how charging is carried out in the powercircuit for a battery of FIG. 11;

FIG. 16 is a diagram showing a relationship between a capacitor voltageand a maximum regenerative electric power of the power circuit for abattery when an allowable input electric power of the battery is set to1 kW;

FIG. 17 is a diagram showing a regenerative electric power regeneratedin the power circuit for a battery when a large vehicle braking force isrequired to be produced in a relatively short period of time;

FIG. 18 is a flow chart of regenerative control for a power circuit fora battery according to Embodiment 4 of the present invention;

FIG. 19 is a flow chart of regenerative control for the power circuitfor a battery according to Embodiment 4 of the present invention;

FIG. 20 is a diagram showing how charging is carried out in accordancewith the flow chart shown in FIGS. 18 and 19;

FIG. 21 is a flow chart of regenerative control for a power circuit fora battery according to Embodiment 5 of the present invention;

FIG. 22 is a flow chart of regenerative control for the power circuitfor a battery according to Embodiment 5 of the present invention;

FIG. 23 is a flow chart of regenerative control for a power circuit fora battery according to Embodiment 6 of the present invention; and

FIG. 24 is a flow chart of regenerative control for the power circuitfor a battery according to Embodiment 6 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1

FIG. 1 is a circuit diagram showing a configuration of a power circuitfor a battery according to Embodiment 1 of the present invention. Asshown in the figure, a battery (first battery group) 1 and a capacitorgroup 2 as a second battery group are connected in series with eachother to constitute a series-connected power supply. The capacitor groupis constituted by capacitors each having a large capacity. For example,an electrical double layer capacitor, an aluminum electrolytic capacitoror the like can be used in the capacitor group. While not illustrated inFIG. 1, an electrical load (not shown; refer to Patent Document 1) suchas an on-vehicle apparatus is connected to the battery 1. Here, in thefollowing description, it is supposed that the battery 1 is an energystorage source, disposed on a low voltage side, of the above-mentionedseries-connected power supply, and the capacitor group 2 is an energystorage source, disposed on a high voltage side, of the above-mentionedseries-connected power supply. Reference numeral 3 designates a DC/DCconverter inserted between the battery 1 and the capacitor group 2. TheDC/DC converter includes a MOSFET (switching element) 31 as an upper armswitch, a MOSFET (switching element) 32 as a lower arm switch, aninductor 33, and a smoothing capacitor 34. Reference numeral 4designates an electric power conversion circuit connected between bothterminals of the series-connected pair of the battery 1 and capacitorgroup 2. The electric power conversion circuit 4 carries out energyshift between the energy of the battery 1 and the capacitor group 2, andthe energy of an electric motor (motor) 9. Reference numeral 10designates an engine. The engine 10 is directly connected to theelectric motor 9 or is mechanically connected to the electric motor 9through a belt or the like to effect power transmission between theengine 10 and the electric motor 9. Reference numeral 5 designates acontroller for issuing an output command signal to the DC/DC converter3. The controller 5 issues a command signal to the MOSFET 31 and theMOSFET 32 in accordance with a voltage across the terminals of thebattery 1, a voltage across the terminals of the smoothing capacitor 34,and an input current to the electric power conversion circuit 4.

Note that while various kinds of configurations are conceivable withrespect to the configuration of the DC/DC converter 3 in addition to theconfiguration shown in FIG. 1, any configuration may be adopted as longas an electric power can be basically transferred between the battery 1and the capacitor group 2. In addition, while the MOSFETs 31 and 32 areused in the DC/DC converter 3, a semiconductor device such as an IGBT ora bipolar transistor may also be used.

Also, while not illustrated in FIG. 1, internal resistances exist in thebattery 1 and the capacitor group 2, respectively. Thus, if a largecurrent is caused to flow through the battery 1 and the capacitor group2, then voltage drops occur due to these internal resistances, and avoltage obtained by subtracting these voltage drops from a total voltageof the battery 1 and the capacitor group 2 is applied to the electricpower conversion circuit 4.

Next, an operation will hereinafter be described. The present inventionrelates to a method of controlling the DC/DC converter 3. Hereinafter,the present invention will be described by giving a certain onecondition as an example. The battery 1 has an output voltage of 12 V andan internal impedance of 8 mΩ. The capacitor group 2 is a capacitorblock in which electrical double layer capacitors each having awithstand voltage of 2.5 V, an internal impedance of 8 mΩ, and anelectrostatic capacity of 100 F. are connected, with three capacitors inparallel and four capacitors in series. Thus, a maximum voltage of thecapacitor group 2 is 10 V. At the start-up, a maximum voltage of 22 Vobtained by adding the voltage 12 V of the battery 1 to the maximumvoltage 10 V of the capacitor group 2 is applied to the electric powerconversion circuit 4. The reason for this is to make the input voltageof the electric power conversion circuit 4 be equal to or higher thanthe battery voltage at start-up to provide a high output, and toincrease a vehicle speed using only the electric motor 9 until the rpmreaches the predetermined motor rpm (about 800 idle rpm in engine rpmand about 2,000 rpm in motor rpm).

FIG. 2 shows a calculation model for determining a control condition forthe DC/DC converter 3 of the present invention. Since an operation timeat start-up is so short as to be about 0.3 seconds, the capacitor group2 is regarded as a power source, and a transient change in the capacitorvoltage is ignored. In the figure, reference symbol η designatesefficiency. Then, it is supposed that the efficiency η is changed withinthe range of 0.975 to 0.9 when the output voltage of the DC/DC converter3 is in the range of 0.5 to 2.0 kW. Also, reference symbol n designatesa boosting ratio, reference symbol ΔV designates a capacitor voltage ofthe capacitor group 2, reference symbol r designates an internalresistance of the capacitor, reference symbol V designates a batteryvoltage of the battery 1, and reference symbol R designates an internalresistance of the battery.

Equations obtained from the calculation model are expressed as follows.$\begin{matrix}{{Vin} = \frac{Vout}{n}} & (1) \\{{Idc} = {\frac{\eta}{n}I_{1}}} & (2) \\{{{R\left( {I_{1} + I_{2}} \right)} + {Vin}} = V} & (3) \\{{Iout} - {Idc} + I_{2}} & (4) \\{{Pdc} = {IdcVout}} & (5) \\{{{R\left( {I_{1} + I_{2}} \right)} + {rI}_{2} + {Vout}} = {V + {\Delta\; V}}} & (6)\end{matrix}$

By solving Equations 1 to 6, it is possible to derive a relationshipbetween the charge voltage (an accumulated voltage in the figure) andthe output voltage (Vout, i.e., the input voltage to the electric powerconversion circuit 4) of the capacitor group 2 for each output of theDC/DC converter 3. The output condition is 4 kW at which a predeterminedmotor output of the electric motor 9 can be obtained.

FIG. 3 shows a relationship between the charge voltage and the outputvoltage of the capacitor group 2 for each output of the DC/DC converter3. In order to achieve the predetermined start-up operation as describedabove, an output voltage (the input voltage to the electric powerconversion circuit 4) of equal to or higher than 10 V is requiredbecause as the motor rpm increases upon start-up, when the voltage islow, the current can not be caused to flow through the electric motor 9due to a back electromotive force generated by the electric motor 9itself and hence it becomes impossible to obtain the motor output. Inaddition, in Embodiment 1 of the present invention, a maximum outputelectric power of the DC/DC converter 3 is 2 kW. From FIG. 3, it isunderstood that when the accumulated voltage is lower than 4 V, it isimpossible to obtain the output voltage of equal to or higher than 10 V.In addition, from the figure, it is also understood that when theaccumulated voltage is 4 V, an output of 2 kW is required for the DC/DCconverter 3.

As described above, in Embodiment 1, it is understood that when theaccumulated voltage is lower than 4 V, it is impossible to obtain thepredetermined output (corresponding to 10 V or more, i.e., 4 kW). InEmbodiment 1, the DC/DC converter 3 is operated for boosting byutilizing a period of time from idle-stop to restart to thereby chargethe capacitor group 2 with electricity, or the capacitor group 2 ischarged with electricity by utilizing the electric power generated fromthe electric motor 9 during traveling of the vehicle. The charge voltageat this time is about 10 V which is near the withstand voltage. When thecapacitor group 2 is charged with electricity using the DC/DC converter3, a period of time required for the charging is about several seconds.However, when the stop/start-up operation is continuously carried out,since it becomes impossible to secure sufficient time to charge thecapacitor group 2 with electricity, the capacitor voltage graduallydecreases. Thus, eventually, the capacitor voltage becomes insufficient,and hence a predetermined motor output can not be obtained. FIG. 4 showsa relationship between the number of times of idle-stop operation and aninitial voltage value of the capacitor group 2 when the idle-stopoperation is continuously carried out from a fully charged state of thecapacitor without recharging. This relationship represents thecalculation results when an electric power of 4 kW is continuouslyoutputted to the electric power conversion circuit 4 for a period oftime of 0.3 seconds. At this time, the DC/DC converter 3 controls itsoutput so that the efficiency of the system constituted by the battery1, the capacitor group 2, and the DC/DC converter 3 becomes maximum.This maximum efficiency control will be described later.

From FIG. 4, it is understood that when the idle-stop operation iscontinuously carried out nine times, the capacitor initial chargingvoltage at the ninth time becomes about 3.2 V, and hence it becomesimpossible to obtain the predetermined motor output. In Embodiment 1 ofthe present invention, as shown in FIG. 5, system control using theengine 10, the electric motor 9, and the electric power conversioncircuit 4 is carried out so that even when the idle-stop operation iscontinuously carried out, the predetermined motor output is obtained.

An operation will hereinafter be described with reference to FIG. 5.First of all, when the vehicle is stopped to stop the engine, acapacitor voltage Vc is detected (Step S1) to judge whether or not thecapacitor voltage Vc is lower than 4 V (first threshold voltage) (StepS2). If it is judged in Step S2 that the capacitor voltage Vc is lowerthan 4 V, then the idle state is maintained (Step S3). Then, the DC/DCconverter 3 is operated while maintaining the idle state as it is, andthe capacitor voltage Vc is boosted to charge the capacitor group 2 withelectricity so that the capacitor voltage Vc becomes equal to or higherthan 4 V (Step S4). On the other hand, if it is judged in Step S2 thatthe capacitor voltage Vc is equal to or higher than 4 V, then the engine10 is stopped (Step S5). When the engine 10 is started with thecapacitor voltage Vc being insufficient (i.e., when the capacitorvoltage Vc is lower than 4 V), since the engine 10 is in the idle statein Step S3, the engine 10 is started without the idle-stop in Step S5.This control method eliminates a fear of the electric motor 9 beingunable to effect start-up due to insufficiency in the voltage. Inaddition, this also solves a problem of increased quantity of noxiousexhaust materials due to the engine ignition at an engine rpm equal toor smaller than the predetermined engine rpm when the motor is startedin a state of voltage insufficiency.

In addition, the voltage of the battery 1 is detected, and the firstthreshold (4 V in Embodiment 1) of the capacitor voltage is adjusted inaccordance with the voltage of the battery 1, whereby the motor start-upafter the idle-stop operation can be carried out with higherreliability. This control method will hereinafter be described.

In the battery 1, the output voltage V in unloading is slightly changedaccording to the state of charge (SOC). In the case of the 12-V battery,this slight change is accompanied with a voltage fluctuation in therange of about ±1 to about ±2 V. If the voltage value of the 12V-battery is fluctuated, then the maximum output electric power of thebattery 1 is also fluctuated. Thus, there is a possibility that thedesired electric power and voltage (4 kW/10 V or more in Embodiment 1)can not be supplied to the electric power conversion circuit 4, andinsufficient motor start-up may result. FIG. 6 shows a relationshipbetween the battery voltage and the threshold voltage of the capacitorgroup 2 at which the rpm of the motor can be increased up to the desiredrpm. The threshold voltage of the capacitor group 2 when the batteryvoltage is 12 V is 4 V, whereas when the battery voltage decreases to 11V, the threshold voltage of the capacitor group 2 increases to 5.5 V.Conversely, the threshold voltage when the battery voltage is 13 Vdecreases to 2.5 V. As described above, in Embodiment 1, when the SOC ofthe battery group 1 is high (the battery voltage is high), the firstthreshold voltage of the capacitor voltage is reduced, while when theSOC of the battery group 1 is low (the battery voltage is low), thefirst threshold of the capacitor voltage is increased, whereby theengine restart-up due to the idle-stop can be surely carried out.

Next, another DC/DC converter control (maximum efficiency control) ofEmbodiment 1 of the present invention will hereinafter be described.FIG. 7 shows a relationship between the capacitor voltage (theaccumulated voltage in the figure) for each output electric power of theDC/DC converter 3 obtained by solving Equations 1 to 6, and the systemefficiency when the battery group 1, the capacitor group 2, and theDC/DC converter 3 are regarded as one power circuit. The outputcondition is 4 kW. As can be seen from the figure, the driving outputcondition of the DC/DC converter 3 under which the maximum efficiency ofthe system is obtained in each capacitor voltage condition varies, andalso an optimal condition exists for each capacitor voltage. This is thereason why the capacitor voltage can not be made a fixed value.

An advantage of starting the engine 10 from the idle-stop state by theelectric motor 9 under the condition permitting the maximum efficiencyis that less calorification takes place in the system as a whole all themore because the same output can be obtained with higher efficiency. Thepower circuit is operated under the maximum efficiency condition, whichresults in that the calorification can be restrained to a minimum, inparticular, the calorification of the capacitor group 2 is greatlysuppressed. The calorification of the capacitor group 2 is suppressed tothereby solve the problem of reduced life due to rise in temperature ofthe capacitor group 2. Moreover, the calorification of the overall powercircuit disposed in an engine room is suppressed to thereby suppress theproblem of calorification of other apparatuses. From FIG. 7, it isunderstood that in a case where the accumulated voltage is 6 V forexample, when the output of the DC/DC converter is 0 W, theinstantaneous calorification is 2,260 W, whereas when the DC/DCconverter 6 is operated at the electric power of 1.5 kW, a quantity ofinstantaneous calorification decreases to 1,530 W. In addition, it isunderstood that, at the point of the accumulated voltage of 10 V, whenthe DC/DC converter 6 is operated at 2 kW, the instantaneouscalorification is 1,260 W, whereas when the DC/DC converter 6 isoperated at 0.5 kW, the instantaneous calorification decreases to 880 W.

Thus, in Embodiment 1 of the present invention, the capacitor voltage isdetected, and the output electric power of the DC/DC converter 3 iscontrolled in accordance with the detected voltage value. FIG. 8 shows arelationship between the capacitor voltage (the accumulated voltage inthe figure) and the output threshold voltage of the DC/DC converter 3 ofEmbodiment 1. The data in FIG. 8 is obtained from FIG. 7. In Embodiment1 of the present invention, since, as shown in FIG. 8, the outputelectric power value of the DC/DC converter 3 can be adjusted inaccordance with the value of the capacitor voltage, the power circuitcan be operated so as to obtain the maximum efficiency.

In Embodiment 1, since the output electric power value of the DC/DCconverter 3 is controlled, the current from the battery 1 to the DC/DCconverter 2, and the voltage of the battery 1 are detected, an outputtarget current value is set in the control circuit 8, and a duty ratioof a gate voltage signal of the MOSFET 32 as the switching element isadjusted on the basis of the comparison of the detected current valuewith the target current value.

While the description has been given with respect to the form in whichthe capacitor group is used as the second energy storage source, it isto be understood that even when the capacitor group is replaced with abattery, the same effects are obtained (in the case of the battery aswell, the output voltage decreases if the discharge is repeatedlycarried out).

Note that, in the above explanation, the effects of the presentinvention have been described under the above-mentioned certaincondition. However, it is needless to mention that, not only in theabove case, if the internal impedances of the battery and the capacitor,and the efficiency of the DC/DC converter change, then the capacitorthreshold voltage (4 V) in the idle-stop condition and the outputcondition of the DC/DC converter corresponding to the capacitor voltagechange accordingly.

In addition, while the description has been given with respect to theoperation in the motor restart after the idle-stop operation (theelectric power of 4 kW is outputted for a period of time of 0.3 sec),even when torque assistance after the motor restart (for about 1 sec) iscarried out, the output of the DC/DC converter is controlled inaccordance with the capacitor voltage which drops with time through thedischarge of the capacitor energy to allow the same effects to beobtained. The torque assistance means that when a vehicle is operatedusing an engine, a motor is also operated simultaneously.

As described above, the power circuit for a battery according toEmbodiment 1 includes a series-connected power supply having the battery1 and the capacitor group 2 connected in series with each other, and theDC/DC converter 3 for shifting the electric power between the battery 1and the capacitor group 2, and between the battery 1 and the electricalload. The voltage Vc of the capacitor group 2, as the energy storagesource disposed on the high voltage side, of the series-connected powersupply is detected, and when the detected voltage Vc is smaller than thepredetermined value (the first threshold voltage, i.e., 4 V in thiscase), the capacitor group 2 is charged with electricity on the basis ofthe boosting operation of the DC/DC converter 3 so that the voltage ofthe capacitor group 2 becomes equal to or higher than the thresholdvoltage (first threshold voltage). As a result, the power circuit for abattery can always output the sufficient electric power.

In addition, the voltage of the capacitor group 2, as the energy storagesource disposed on the high voltage side, of the series-connected powersupply is detected, and when the detected voltage is smaller than thepredetermined value (first threshold voltage), the rpm of the engine 10is maintained at the idle rpm without carrying out the engine stopoperation. As a result, it is possible to eliminate the problem of theelectric motor 9 being unable to effect engine start-up.

In addition, the voltage of the capacitor group 2, as the energy storagesource disposed on the high voltage side, of the series-connected powersupply is detected, and when the detected voltage is smaller than thepredetermined value (first threshold voltage), the running of the engineis maintained, and after the capacitor group 2 is charged withelectricity on the basis of the boosting operation of the DC/DCconverter 3 so that the capacitor voltage becomes equal to or higherthan the threshold voltage (first threshold voltage), the engine 10 isstopped. As a result, even when the stop/start-up operation (idle-stopoperation) is continuously carried out, sufficient electric power can besupplied to the electric motor 9 at start-up, and hence the engine rpmcan be increased up to the predetermined rpm by the electric motor 9.Thus, it is possible to solve the problem of increased quantity ofnoxious exhaust materials due to the gasoline ignition in the lowrevolution range at start-up. In addition, it is possible to eliminatethe problem of the electric motor 9 being unable to effect enginestart-up.

Also, the voltage of the battery 1 as the first energy storage source,and the voltage of the capacitor group 2 as the second energy storagesource are detected, and the threshold voltage (first threshold voltage)of the capacitor group 2 serving as a criterion for determining whetheror not to restart the engine is adjusted in accordance with the voltagevalue of the battery 1 as the energy storage source disposed on the lowvoltage side. Thus, when the SOC of the battery 1 is high (the batteryvoltage is high), the threshold voltage of the capacitor is reduced,while when the SOC of the battery 1 is low (the battery voltage is low),the threshold voltage of the capacitor group 2 is increased. As aresult, it is possible to surely carry out the engine restart-up basedon the idle-stop.

Also, when the engine is started from the idle-stop state (motor stopstate) by the motor, the voltage of the capacitor group 2, as the energystorage source disposed on the high voltage side, of theseries-connected power supply is detected, and the output electric powerof the DC/DC converter 3 is changed in accordance with the detectedcapacitor voltage. Thereby, it becomes possible to operate the overallpower circuit system for a battery at a maximum efficiency, and aquantity of calorification of the overall system can be restrained to aminimum. In particular, the reduction in the life span of the capacitorgroup 2 due to rise in temperature caused by the calorification, andalso an influence of the calorification exerted on other apparatuses canbe suppressed.

Embodiment 2

FIG. 9 is a flow chart showing a flow of an operation of a power circuitfor a battery according to Embodiment 2 of the present invention. Notethat since a configuration of the power circuit for a battery accordingto Embodiment 2 of the present invention is the same as that ofEmbodiment 1 shown in FIG. 1, the reference should be made to FIG. 1 inthis regard, and a detailed description is omitted here.

As shown in FIG. 9, in Embodiment 2, first of all, when an enginestart-up command is issued in Step S10, in Step S11, it is judgedwhether or not the voltage across the terminals of the battery 1 ishigher than a second threshold voltage V_(TH2) (e.g., 8.0 V). If it isjudged in Step S11 that the voltage across the terminals of the battery1 is higher than the second threshold voltage V_(TH2), then theoperation proceeds to Step S12. The maximum efficiency control describedin Embodiment 1 is carried out, and in Step S15, the engine is started.At this time, a judgment on the voltage across the terminals of thebattery 1 in Step S11 is carried out at predetermined time intervalswhile carrying out the maximum efficiency control. On the other hand, ifit is judged in Step S11 that the voltage across the terminals of thebattery 1 is equal to or lower than the second threshold voltage V_(TH2)(e.g., 8.0 V), then in Step S13, the control mode is changed over to thebattery current minimum control. Then, in Step S14, the first thresholdvoltage V (its initial value in Embodiment 2 is 4 V) of the capacitorgroup 2, which serves as the condition for determining whether or not tostop the vehicle to stop the engine, is increased. Then, in Step S15,the engine is started.

Note that while in the above explanation, the description has been givenwith respect to the example in which the processing in Steps S13 and theprocessing in S14 are continuously executed, the present invention isnot limited to this case. That is to say, the judgment on the conditionfor determining whether or not the processing in Step S14 is to beexecuted may be carried out before the processing in Step S14. That is,as the condition for increasing the capacitor threshold voltage, it maybe set in advance that the processing in Step S14 is executed when theterminal voltage V of the battery 1 is higher than a third thresholdvoltage (e.g., 8.0 V).

Note that while described in Embodiment 1 as well and not illustrated inFIG. 1, the internal resistances exist in the battery 1 and thecapacitor group 2, and hence if a large current is caused to flowthrough the battery 1 and the capacitor group 2, voltage drop or voltagerise occurs due to these internal resistances. An electrical load(illustration thereof is omitted; refer to Patent Document 1) such as anon-vehicle apparatus is connected to the battery 1, and hence there is apossibility that if the voltage across the terminals of the battery 1abruptly decreases, this decrease may exert adverse influences on theelectrical load. As for the electrical load connected to a 12 V-battery(its charge voltage is 14 V) which is generally used, there are manyproducts for which the operation is ensured when the battery voltage isequal to or higher than 8 V. In addition, in the system using a 36V-battery (its charge voltage is 42 V) which is anticipated to come intowide use in the future, a standard is being established which stipulatesthat the lowest voltage of a voltage across terminals of a battery bekept within the range of 21 to 25 V, and the highest voltage thereof bekept within the range of 51 to 55 V. Thus, when the electrical load isconnected to the battery 1, it is necessary to limit the battery currentso as to prevent the voltage across the terminals of the battery 1 frombecoming equal to or smaller than a certain reference voltage value(about ⅔ of a nominal voltage value).

Since in Embodiment 2, the battery voltage changes in accordance withthe output electric power of the DC/DC converter 3, it is necessary toobtain a relationship between the output electric power of the DC/DCconverter 3 and the battery voltage. As for a path through which theelectric power of the power circuit for a battery according to thepresent invention is outputted, there are a path through which theelectric power is outputted from the battery 1 via the DC/DC converter 3(hereinafter referred to as “output P₁” when applicable), and a paththrough which the electric power is outputted from the battery 1 via thecapacity group 2 (hereinafter referred to as “output P₂” whenapplicable). A total electric power (P₁+P₂) of the outputs P₁ and P₂becomes an electric power inputted to the electric power conversioncircuit 4. When the electric motor 9 generates an electric power withwhich the battery 1 and the capacitor group 2 are in turn charged, theoutputs P₁ and P₂ are negative values.

Next, the outputs P₁ and P₂, and an output voltage of the battery 1 atthat time (an input voltage to the DC/DC converter) V_(in) arecalculated using the calculation model shown in FIG. 2. When a voltageacross the terminals of the battery 1 in an unloading state is assignedV, a voltage across the terminals of the capacitor group 2 in anunloading state is assigned ΔV, the internal resistance of the battery 1is assigned R, the internal resistance of the capacitor group 2 isassigned r, a current caused to flow through the battery 1 is assignedI_(B), a current caused to flow through the capacitor group 2 isassigned I₂, an input current to the DC/DC converter 3 is assigned I₁,and an electric power conversion efficiency of the DC/DC converter 3 isassigned η, the outputs P₁ and P₂ are expressed as follows.P ₁=(V _(in) +ΔV−r×I ₂)×I ₂P ₂ =V _(in) ×I ₁×ηV _(in) =V−R×I _(B)I _(B) =I ₁ +I ₂

From the equations described above, the current I_(B) caused to flowthrough the battery 1 when the output electric power of the powercircuit for a battery is P is expressed as follows. $\begin{matrix}{I_{B} = {\frac{\left( {V + {\Delta\; V}} \right) + {\left\{ {{\left( {1 - \eta} \right)R} + {2r}} \right\} I_{1}}}{2\left( {R + r} \right)} -}} \\{\frac{\sqrt{\left\{ {\left( {V + {\Delta\; V}} \right) - {{R\left( {1 + \eta} \right)}I_{1}}} \right\}^{2} - {4\left( {R + r} \right)\left\{ {P + {\left( {{R \cdot I_{1}^{2}} - {V \cdot I_{1}}} \right)\eta}} \right\}}}}{2\left( {R + r} \right)}}\end{matrix}$

From the equation described above, it is understood that even if theoutput electric power P of the power circuit for a battery is fixed, thebattery current depends on the input current I₁, to the DC/DC converter3 and the voltage ΔV across the terminals of the capacitor group 2 in anunloading state.

As one example, there is shown in FIG. 10 a relationship between theoutput of the DC/DC converter 3 and the battery terminal voltage whenthe voltage V across the terminals of the battery 1 in an unloadingstate is 12 V, the voltage ΔV across the terminals of the capacitorgroup 2 in an unloading state is 6 V, the internal resistance R of thebattery 1 is 9.6 mΩ (20% increase from 8 mΩ due to the degradation orthe like), and the initial resistance r of the capacitor group 2 is 10.7mΩ, and under this condition, the power circuit for a battery outputsthe electric power of 4 kW. In the figure, there are shown the maximumefficiency running point described in Embodiment 1, and the batterycurrent minimum point described in Embodiment 2.

From FIG. 10, it is understood that the DC/DC converter output and thebattery voltage at the maximum efficiency running point are 1,500 W and7.9 V, respectively, whereas at the battery current minimum point thebattery voltage is 8.3 V and thus about 5% higher. As described above,it is understood that when the results of detection of the batteryvoltage show that the battery voltage becomes equal to or lower than thepreset second threshold voltage (e.g., 8.0 V), the control mode ischanged over from the maximum efficiency control to the battery currentminimum control (Steps S11 and S13 in FIG. 9), whereby it becomespossible to suppress a decrease in output voltage of the battery.

In addition, when the control mode is changed over to the batterycurrent minimum control, the first threshold voltage (its initial valuein Embodiment 2 is 4 V) of the capacitor group 2 serving as thecondition for determining whether or not to stop a vehicle to stop theengine 10 is increased (Step S14 in FIG. 9). The first threshold voltageof the capacitor group 2 is reset in accordance with the detectedbattery voltage. As a result, when a vehicle is stopped next time, thefirst threshold voltage of the capacitor group 2 serving as the enginestop condition becomes (4 V+α)(α>0). Thus, it is possible to prevent theoutput voltage of the battery 1 from decreasing.

Even when the internal resistances of the battery 1 and the capacitancegroup 2 are increased due to the degradation or the like of the battery1 and the capacitor group 2 to deteriorate the battery performance andthe capacitor performance, it is possible to suppress the decrease ofthe output voltage of the battery 1 at the time of outputting thepredetermined electric power. Thus, the start-up operation of the engine10 can be surely carried out without exerting adverse influences onother on-vehicle apparatuses connected to the battery 1.

Note that, in Embodiment 2, the same voltage value of 8.0 V serves asboth the condition (second threshold voltage) for changing over thecontrol mode from the maximum efficiency control to the battery currentminimum control, and the condition (third threshold voltage) forincreasing the capacitor threshold voltage. However, the presentinvention is not limited to this case. Thus, it is to be understood thatthe same effects can be obtained even when different values are set asthe second and third threshold voltages.

As described above, similarly to Embodiment 1 described above, the powercircuit for a battery according to Embodiment 2 includes theseries-connected power supply having the battery 1 and the capacitorgroup 2 connected in series with each other, and the DC/DC converter 3for shifting the electric power between the battery 1 and the capacitorgroup 2, and between the battery 1 and the electrical load. Thus, thevoltage Vc of the capacitor group 2, as the energy storage sourcedisposed on the high voltage side, of the series-connected power supplyis detected, and when the detected voltage Vc is lower than thepredetermined value (the first threshold voltage, i.e., 4 V in thiscase), the capacitor group 2 is charged with electricity on the basis ofthe boosting operation of the DC/DC converter 3 so that the voltage ofthe capacitor group 2 becomes equal to or higher than the thresholdvoltage (first threshold voltage). Hence, the power circuit for abattery can always output sufficient electric power.

In addition, similarly to Embodiment 1, the voltage of the capacitorgroup 2, as the energy storage source disposed on the high voltage side,of the series-connected power supply is detected, and when the detectedvoltage is smaller than the predetermined value (first thresholdvoltage), the rpm of the engine 10 is maintained at the idle rpm withoutcarrying out the engine stop operation. Hence, it is possible toeliminate the problem of the electric motor 9 being unable to effectengine start-up.

In addition, similarly to Embodiment 1, the voltage of the capacitorgroup 2, as the energy storage source disposed on the high voltage side,of the series-connected power supply is detected, and when the detectedvoltage is smaller than the predetermined value (first thresholdvoltage), the engine rpm is maintained, and after the capacitor group 2is charged with electricity on the basis of the boosting operation ofthe DC/DC converter 3 so that the capacitor voltage becomes equal to orhigher than the threshold voltage (first threshold voltage), the engineis stopped. Thus, even when the stop/start-up operation (idle-stopoperation) is continuously carried out, the electric power can besufficiently supplied to the electric motor 9 at start-up, and hence theengine rpm can be increased to the predetermined rpm by the electricmotor 9. As a result, it is possible to solve the problem of increasedquantity of noxious exhaust materials due to the gasoline ignition inthe low rpm range at start-up. In addition, it is possible to solve theproblem of the electric motor 9 being unable to effect engine start-up.

Moreover, in Embodiment 2, the voltage value of the battery 1 as thefirst energy storage source is detected, and when, at the time ofoutputting large electric power, e.g., upon restarting the engine 10,the battery voltage becomes smaller than the preset reference voltagevalue (second threshold voltage), the DC/DC converter 3 is controlled sothat the battery current becomes minimum. Thus, it is possible tosuppress the voltage drop due to the internal resistance of the battery1, and hence it is possible to eliminate adverse influences exerted onother on-vehicle apparatuses connected to the battery 1.

Also, when the results of detection of the voltage value of the battery1 as the first energy storage source show that the battery voltagebecomes smaller than the preset reference voltage value (third thresholdvoltage), the first threshold voltage is increased and this is reflectedon the idle-stop condition from the next time onward. Thus, it ispossible to suppress reduction of the battery voltage at start-up of themotor, and hence it is possible to eliminate adverse influences exertedon other on-vehicle apparatuses connected to the battery 1.

Embodiment 3

FIG. 11 is a circuit diagram showing a configuration of a power circuitfor a battery according to Embodiment 3 of the present invention. FIG.12 is a detailed block diagram of a controller shown in FIG. 11. FIGS.13 and 14 are a flow chart of regenerative control in FIG. 11. FIG. 15is a diagram showing how charging is carried out in a power circuit fora battery shown in FIG. 11.

As shown in FIG. 11, the power circuit for a battery includes: a battery1 as a first energy storage source; a capacitor 2 which is connected inseries with the battery 1 and which serves as a second energy storagesource having an allowable input current larger than that of the battery1; a DC/DC converter 3 inserted between the battery 1 and the capacitor2; an electric power conversion circuit 4 connected between terminals ofa series-connected body of the battery 1 and the capacitor 2; and acontroller 5 for controlling the DC/DC converter 3 and the electricpower conversion circuit 4.

The battery 1 is a plumbic acid battery having a rated voltage of 12 Vand an equivalent series resistance of 8 mΩ. The battery 1 has thecharacteristic of the allowable input electric power=PBMAX(W). Since thebattery 1 is degraded if rapidly charged with a large electric power, anallowable input electric power PBMAX corresponding to a batterytemperature and a state of charge (SOC) is set in the battery 1. Sincethe voltage of the battery 1 does not largely change, an allowable inputcurrent IBMAX is set therefor instead of the allowable input electricpower PBMAX. The allowable input electric power PBMAX of the plumbicacid battery is about 100 W/kg.

The capacitor 2 is an electrical double layer capacitor, an aluminumelectrolytic capacitor, or the like having a large electric capacity. Anallowable input electric power PCMAX(W) of the capacitor 2 is largerthan that of the plumbic acid battery, and is about 1,000 W/kg. InEmbodiment 3, used as the capacitor 2 is a capacitor block in which theelectrical double layer capacitors each having an allowable appliedvoltage (VCMAX) of 2.5 V, an equivalent series resistance (r) of 8 mΩ,and an electrostatic capacity (C) of 100 F are connected, with threecapacitors in parallel and fifteen capacitors in series. An allowableapplied voltage of the capacitor 2 is 37.5 V.

The DC/DC converter 3 includes an upper arm switching element 6 a as anupper arm switch, a lower arm switching element 6 b as a lower armswitch, a choke coil inductor 7, and a smoothing capacitor 8. The DC/DCconverter 3 constitutes a bidirectional boosting/deboosting D.C. choppercircuit, and the electric power is shifted between the battery 1 and thecapacitor 2 through the DC/DC converter 3. Each of the switchingelements 6 a and 6 b is constituted by a MOSFET.

The DC/DC converter 3 is controlled in a manner as will be describedbelow to carry out the electric power conversion. In the followingdescription, the electric power shift from the battery 1 towards thecapacitor 2 (hereinafter referred to as a boosting mode DC/DC converteroperation) is given as an example.

The upper arm switching element 6 a is turned OFF and the lower armswitching element 6 b is turned ON to cause a current to flow from thebattery 1 to the choke coil inductor 7. Next, the lower arm switchingelement 6 b is turned OFF and at the same time, the upper arm switchingelement 6 a is turned ON to apply the current caused to flow through thechoke coil inductor 7 across the terminals of the capacitor 2 throughthe upper arm switching element 6 a. The electric power of the battery 1is supplied to the capacitor 2 by repeatedly carrying out thisoperation. The output current of the DC/DC converter 3 can be changed bychanging an ON-time ratio between the upper arm switching element 6 aand the lower arm switching element 6 b.

The electric power shift from the capacitor 2 towards the battery 1(hereinafter referred to as a deboosting mode DC/DC converter operation)can be carried out by performing the operation reverse to that in theabove description.

The MOSFET as the switching element receives as its input a signal toturn ON/OFF its gate to thereby carry out the switching operation.

The electric power conversion circuit 4 carries out the electric powershift between the series-connected body of the battery 1 and thecapacitor 2, and an electric motor 9. The electric motor 9 is connectedto an axle 27 coupled to an engine 10. At start-up, the D.C. electricpower from the battery 1 and the capacitor 2 is converted into an A.C.electric power by the electric power conversion circuit 4 to rotate theaxle 27 with the electric motor 9 as a motor. For braking, an A.C.electric power generated from the electric motor 9 serving as a powergenerator is converted into a D.C. electric power by the electric powerconversion circuit 4 to charge the battery 1 and the capacitor 2 withelectricity. The A.C. electric power acts as a braking force against therotation of the axle 27. The axle 27 is provided with a brake mechanism26 for applying the brake on the rotation of the axle 27. The brakemechanism 26 is provided with a mechanical brake (not shown) formechanically applying the brake on the rotation of the axle 27 inaccordance with a command issued from the controller 5.

There is further provided a brake pedal 28, as a braking command unit,for issuing a brake command in accordance with which the speed of thevehicle is decelerated. The brake command is inputted in the form of abraking force PF to the controller.

The controller 5, as shown in FIG. 12, has a DC/DC converter controlunit 11. The DC/DC converter control unit 11 includes input currentcalculation means 13, regeneration-enabling electric power calculationmeans 14, allowable input current calculation means 15, DC/DC convertercontrol means 16, and mechanical brake control means 18. The controller5 is constituted by a microcomputer having a CPU, a RAM, a ROM and aninterface circuit.

The power circuit for a battery, as shown in FIG. 11, further includes abattery voltmeter 20 as a first voltmeter for measuring a voltage VBdeveloped across the terminals of the battery 1, a capacitor voltmeter21 as a second voltmeter for measuring a voltage VC developed across theterminals of the capacitor 2, a thermometer 24 for measuring atemperature TB of the battery 1, and a vehicle speed sensor 25 formeasuring a vehicle speed Sv.

An electrical load 22 such as an on-vehicle apparatus is connected tothe battery 1.

Next, a description will hereinafter be given with respect to aconfiguration of the controller 5 of the power circuit for a batteryshown in FIG. 12.

The input current calculation means 13 calculates a regenerative energyPG (W) on the basis of the vehicle speed Sv(km/hr) from the vehiclespeed sensor 25, and a braking force PF(N) from the brake pedal 28. Theregenerative energy PG is a value which is obtained by converting abraking energy required to apply the brake on a vehicle traveling at thevehicle speed Sv with the predetermined braking force PF into a quantityof electricity. Moreover, the input current calculation means 13 obtainsan input current I(A) on the basis of the voltage VB developed acrossthe terminals of the battery 1, the voltage VC developed across theterminals of the capacitor 2, and the regenerative energy PG.

On the other hand, the regeneration-enabling electric power calculationmeans 14 calculates the SOC(%) of the battery 1 on the basis of thevoltage VB developed across the terminals of the battery 1. The batterySOC is a value corresponding to the terminal voltage VB of the battery1. This value is stored as table data in the regeneration-enablingelectric power calculation means 14.

Moreover, the regeneration-enabling electric power calculation means 14calculates the battery allowable input electric power PBMAX(W) from thebattery SOC. When the battery temperature TB is high, the batteryallowable input electric power PBMAX becomes less, and when the batterySOC is large, the battery allowable input electric power PBMAX alsobecomes less.

Moreover, the regeneration-enabling electric power calculation means 14calculates the maximum regenerative electric power PINVMAX(0)(W) fromthe battery allowable input electric power PBMAX.

Furthermore, the regeneration-enabling electric power calculation means14 calculates the maximum regenerative electric power PINVMAX(100)(W)when the DC/DC converter 3 is operated at maximum to carry out theelectric power conversion on the basis of the battery allowable inputelectric power PBMAX and a preset DC/DC converter maximum outputPDMAX(W).

The allowable input current calculation means 15 obtains an allowableinput current IBMAX(A) of the battery 1 on the basis of the batteryallowable input electric power PBMAX and the terminal voltage VB of thebattery 1.

The DC/DC converter control means 16, when the regenerative energy PG islarger than the maximum regenerative electric power PINMAX(0),calculates a DC/DC converter operation quantity PD on the basis of theregenerative electric power PINVMAX(0), the battery voltage VB, thecapacitor voltage VC and the battery maximum current IBMAX.

Moreover, the DC/DC converter control means 16 obtains a ratio n betweena boosted quantity and a deboosted quantity from the DC/DC converteroperation quantity PD. In this connection, when the current of thebattery 1 is assigned IB*, the electric power PD(W) which is shiftedfrom the battery 1 to the capacitor 2 through the DC/DC converter 3 isexpressed as PD=VD×(IC−IB). The DC/DC converter control means 16 obtainsVD=PD/(IC−IB) to obtain a ratio n=VD/VB between a boosted quantity and adeboosted quantity.

Furthermore, the DC/DC converter control means 16 obtains a period ofturn-ON/OFF of the switching means so as to obtain the ratio n between aboosted quantity and a deboosted quantity, and under this condition,operates the DC/DC converter.

As described above, in this case, the DC/DC converter control means 16shifts the electric power from the battery 1 to the capacitor 2.

The DC/DC converter control means 16, when the regenerative energy PG isequal to or less than the maximum regenerative electric powerPINVMAX(0), calculates the DC/DC converter operation quantity PD on thebasis of the regenerative electric power PINVMAX(0), the battery voltageVB, the capacitor voltage VC and the battery maximum current IBMAX.

In this case, the DC/DC converter control means 16 shifts the electricpower from the capacitor 2 to the battery 1.

The mechanical brake control means 18 compares the regenerative energyPG with the maximum regenerative electric power PINVMAX(100) If it isjudged that the regenerative energy PG is larger than the maximumregenerative electric power PINVMAX(100), then the mechanical brakecontrol means 18 obtains a difference ΔPA between the regenerativeenergy PG and the maximum regenerative electric power PINVMAX(100) toconvert the difference ΔPA into a mechanical brake operation quantityMF.

Moreover, the mechanical brake control means 18 operates the mechanicalbrake on the basis of the mechanical brake operation quantity MF toapply the brake on the vehicle.

Next, referring to FIGS. 13 and 14, a description will hereinafter begiven with respect to a procedure of regeneration control of the powercircuit for a battery.

In Step (hereinafter abbreviated as S when applicable) 101, the inputcurrent calculation means 13 acquires data of the vehicle speed Sv(km/hr) from the vehicle speed sensor 25 to judge whether or not thevehicle speed Sv is zero. The regeneration control is completed when thevehicle speed is zero, because the vehicle is in a stop state. On theother hand, when the vehicle speed is not zero, the operation proceedsto S102.

In S102, the input current calculation means 13 acquires data of thebraking force PF(N) from the brake pedal 28 to judge whether or not thebraking command is issued. When the braking force is zero, noregenerative energy is generated because no braking is tried to beapplied. Thus, the regeneration control is completed. If it is judged inS102 that the braking command is issued, then the operation proceeds toS103.

In S103, the input current calculation means 13 calculates theregenerative energy PG(W) on the basis of the vehicle speed Sv and thebraking force PF. Then, the input current calculation means 13 obtainsthe input current I(A) on the basis of the regenerative energy PG, theterminal voltage VB(V) of the battery, and the terminal voltage VC(V) ofthe capacitor.

In S104, the allowable input current calculation means 15 calculates theSOC(%) of the battery from the terminal voltage VB (V) of the battery.

In S105, the allowable input current calculation means 15 calculates thebattery allowable input electric power PBMAX(W) from the batterytemperature TB(° C.) and the SOC of the battery. Also, the allowableinput current calculation means 15 calculates the battery allowableinput current IBMAX(A) from the battery allowable input electric powerPBMAX and the battery terminal voltage VB.

In S106, the regeneration-enabling electric power calculation means 14compares the input current I with the battery allowable input currentIBMAX. If it is judged in S106 that the input current I is larger thanthe battery allowable input current IBMAX, then the operation proceedsto S107. On the other hand, if it is judged in S106 that the inputcurrent I is equal to or smaller than the battery allowable inputcurrent IBMAX, the regeneration control is completed.

In S107, the regeneration-enabling electric power calculation means 14obtains the capacitor input electric power PC(IBMAX) from the batteryallowable input current IBMAX and the terminal voltage Vc of thecapacitor. Also, the regeneration-enabling electric power calculationmeans 14 calculates the maximum regenerative electric power PINVMAX(0)when no DC/DC converter is driven from the battery allowable inputelectric power PBMAX and the capacitor input electric power PC(IBMAX).

In S108, the regeneration-enabling electric power calculation means 14obtains a reinforcement maximum regenerative electric power PINVMAX(100)from Equation 1 on the basis of the battery allowable input electricpower PBMAX, the preset maximum driving electric power PDMAX of theDC/DC converter, and the battery terminal voltage VB.${P_{INVMAX}(100)} = {{\frac{V_{B} + V_{C}}{V_{B}}P_{BMAX}} + {\frac{V_{C}}{V_{B}}P_{DMAX}}}$

In S109, the DC/DC converter control means 16 compares the regenerativeenergy PG with the reinforcement maximum regenerative electric powerPINVMAX(100). If it is judged in S109 that the regenerative energy PG islarger than the reinforcement maximum regenerative electric powerPINVMAX(100), then the operation proceeds to S110. On the other hand, ifit is judged in S109 that the regenerative energy PG is equal to orsmaller than the reinforcement maximum regenerative electric powerPINVMAX(100), then the operation proceeds to S113.

In S110, the DC/DC converter control means 16 sets the reinforcementmaximum regenerative electric power PINVMAX(100) as the regenerativeoperation quantity PINV. At the same time, the mechanical brake controlmeans 18 obtains a difference ΔPA between the regenerative energy PG andthe reinforcement maximum regenerative electric power PINVMAX(100).

In S111, the mechanical brake control means 18 obtains a mechanicalbrake operation quantity MF from the difference ΔPA.

In S112, the mechanical brake control means 18 operates the brakemechanism 26 on the basis of the mechanical brake operation quantity MFto apply the braking force to the vehicle. Then, the operation proceedsto S114.

In S113, the DC/DC converter control means 16 sets the regenerativeenergy PG as the regenerative operation quantity PINV. Then, theoperation proceeds to S114.

In S114, the DC/DC converter control means 16 obtains a DC/DC converterconversion electric power PD from Equation 2 on the basis of theregenerative operation quantity PINV, the battery allowable inputelectric power IBMAX, the battery terminal voltage VB, and the terminalvoltage VC of the capacitor.$P_{D} = {\frac{V_{B}}{V_{C}}\left\{ {P_{INV} - {\left( {V_{B} + V_{C}} \right)I_{BMAX}}} \right\}}$

In S115, the DC/DC converter control means 16 obtains a DC/DC converteroutput voltage VOUT from the DC/DC converter conversion electric powerPD.

In S116, the DC/DC converter control means 16 compares the DC/DCconverter output voltage VOUT with the capacitor allowable appliedvoltage VCMAX. If it is judged in S116 that the DC/DC converter outputvoltage is lower than the capacitor allowable applied voltage VCMAX,then the operation proceeds to S117. On the other hand, if it is judgedin S116 that the DC/DC converter output voltage VOUT is equal to orhigher than the capacitor allowable applied voltage VCMAX, then theoperation proceeds to S118.

In S117, the DC/DC converter control means 16 obtains the ratio nbetween the boosted quantity and the deboosted quantity from theconverter output voltage VOUT and the battery voltage.

In S118, the DC/DC converter control means 16 obtains the ratio nbetween the boosted quantity and the deboosted quantity from thecapacitor allowable applied voltage VCMAX and the battery voltage VB.

In S119, the DC/DC converter control means 16 drives the DC/DC converterto shift the electric power from the battery to the capacity to chargethe capacity with electricity (the electric power conversion by theDC/DC converter in this direction is referred to as a boosting modeDC/DC converter operation when applicable) Then, the operation isreturned back to S101.

Next, referring to FIG. 15, a description will hereinafter be given withrespect to how charging is carried out while the brake is applied to thevehicle. A vehicle braking force shown in FIG. 15, for example, isgenerated when the vehicle is decelerated at about a fixed accelerationfrom a certain speed.

In a section A, as described above, the DC/DC converter is operated in aboosting mode at a maximum output to thereby increase the reinforcementregenerative electric power. Thus, the regenerative electric powerincreases as the voltage of the capacitor increases. At this time, theelectric power which cannot be regenerated by the power circuit for abattery is consumed by the mechanical brake.

In a section B, when the reinforcement regenerative electric powerbecomes larger than the regenerative energy consumed by the vehiclebraking, no mechanical brake is operated. Thus, the overall vehiclebraking force is converted into the electric power for generation in theelectric motor, which makes it possible to regenerate all the kineticenergies. At this time, the output of the DC/DC converter is controlledso that the charging electric power for the battery becomes theallowable input electric power PBMAX.

In a section C, the output of the DC/DC converter becomes zero. Thecharging electric power for the battery and the capacitor charges inaccordance with a ratio in voltage between the battery and thecapacitor.

A description will hereinafter be given with respect to such an increasein the regenerative electric power using the power circuit for a batterywith reference to FIG. 16. FIG. 16 shows a relationship between thecapacitor voltage VC and the maximum regenerative electric power PINV ofthe power circuit for a battery when the allowable input electric powerPBMAX of the battery 1 is set to 1 kW.

For example, the regenerative electric power when the DC/DC converter 3is operated at 1 kW is, when the capacitor voltage Vc is equal to thebattery voltage VB, 1.5 times as large as that when no DC/DC converter 3is operated, and is, when the capacitor voltage Vc is 3 times as largeas the battery voltage VB, 1.75 times as large as that when no DC/DCconverter 3 is operated.

In addition, the regenerative electric power when the DC/DC converter 3is operated at 2 kW is, when the capacitor voltage VC is equal to thebattery voltage VB, twice as large as that when no DC/DC converter 3 isoperated, and is, when the capacitor voltage Vc is 3 times as large asthe battery voltage VB, 2.5 times as large as that when no DC/DCconverter 3 is operated.

As described above, the regenerative electric power of the power circuitfor a battery further increases as the output of the DC/DC converter 3further increases. As a result, the electric power is shifted from thebattery to the capacitor through the DC/DC converter 3 duringregeneration of the energy to thereby increase the regenerative electricpower of the power circuit for a battery.

FIG. 17 shows the regenerative electric power regenerated in the powercircuit for a battery when a large vehicle braking force is required fora relatively short period of time. The charge current of the capacitorwhen no DC/DC converter 3 is operated is determined on the basis of theallowable charge current of the battery. Thus, since the charge electricpower for the capacitor can not increase, a quantity of energy able tobe regenerated in the power circuit for a battery does not become solarge. On the other hand, if the electric power is shifted from thebattery 1 to the capacitor 2 through the DC/DC converter 3, then thecharge electric power for the capacitor 2 increases. Hence, it becomespossible to increase the regenerative electric power for the powercircuit for a battery. The reason that the charge electric power of thecapacitor 2 increases with time is that since the charging of thecapacitor 2 increases the voltage, the allowable input power to thecapacitor increases accordingly.

In the power circuit for a battery of the present invention, sinceduring the braking of the automobile, the electric power is shifted fromthe battery to the capacitor having input electric power density largerthan that of the battery through the DC/DC converter, it is possible toincrease the charge electric power for the battery and the capacitor.

In addition, since the battery is charged with an electric power equalto or less than the allowable input electric power of the battery, it ispossible to prevent the charging in an over-power state of the battery,and it is also possible to lengthen the life of the battery.

Moreover, since the capacitor is charged with the voltage equal to orlower than the allowable applied voltage of the capacitor, it ispossible to prevent the degradation of the capacitor due to anover-voltage and it is also possible to lengthen the life of thecapacitor.

The power circuit for a battery controls the electric power generated bythe electric motor so that the generated electric power becomes equal toor less than the electric power obtained by adding the allowable inputelectric power of the first energy storage source and the allowableinput electric power of the second energy storage source. Hence, thebraking energy during deceleration of the vehicle can be regeneratedwith high efficiency, and thus the rate of fuel consumption of thevehicle can be improved.

At that, with respect to the configuration of the DC/DC converter 3,there are conceivable various configurations in addition to theconfiguration shown in FIG. 11. However, any configuration may beadopted as long as the electric power is basically shifted between thebattery 1 and the capacitor 2 with the configuration concerned. Inaddition, the MOSFETs are used as the switching elements of the DC/DCconverter 3. However, a semiconductor device such as an IGBT or abipolar transistor may also be used.

Note that while in Embodiment 3, the electrical double layer capacitorsare used as the second energy storage source, even when an aluminumelectrolytic capacitor is used, the same effects can be obtained.

In addition, when the plumbic acid battery is used as the first energystorage source, even if the battery having a large allowable inputelectric power is used as the second energy storage source, similarly,the regenerative electric power can be increased. For example, anickel-cadmium battery, a nickel-hydrogen battery, or a lithium ionbattery may be used.

Embodiment 4

FIGS. 18 and 19 show a flow chart of regenerative control of a powercircuit for a battery according to Embodiment 4 of the presentinvention. A configuration of the power circuit for a battery accordingto Embodiment 4 is the same as that of FIGS. 11 and 12. The flow chartof FIGS. 18 and 19 is the same as that of FIGS. 13 and 14 except thatSteps S201 to S203 are newly added to the flow chart of FIGS. 13 and 14.

If it is judged in S106 that the input current is larger than thebattery allowable input current IBMAX, then the operation proceeds toS107. On the other hand, if it is judged in S106 that the input currentI is equal to or smaller than the battery allowable input current IBMAX,then the operation proceeds to S201.

In S201, similarly to S107, the DC/DC converter control means 16 obtainsa maximum regenerative electric power PINVMAX(0).

In S202, the DC/DC converter control means 16 obtains a difference ΔPBbetween the maximum regenerative electric power PINVMAX(0) and theregenerative energy PG.

In S203, the DC/DC converter control means 16 drives the DC/DC converteron the basis of the difference ΔPB to shift the electric power from thecapacitor to the battery in order to charge the battery with electricity(the electric power conversion by the DC/DC converter in this directionis referred to as a deboosting mode DC/DC converter operation).

Next, a description will hereinafter be given with respect to howcharging is carried out while the brake is applied to the vehicle withreference to FIG. 20. A vehicle braking force shown in FIG. 20, forexample, is generated when the vehicle is decelerated from a certainspeed about at a fixed acceleration.

Sections A and B are the same as those of FIG. 15. In a section C, theDC/DC converter is operated in the deboosting mode to shift the electricpower from the capacitor to the battery so that the input current to thebattery agrees with the allowable input electric power IBMAX. In asection D, the energy stored in the capacitor does not increase. That isto say, comparing this case with the case of FIG. 15, the energy storedin the capacitor in this case is less than that in the case of FIG. 15by ΔEc.

This deboosting mode operation makes it possible to reduce a quantity ofcharge energy for the capacitor as compared with the case in FIG. 15where the DC/DC converter is not operated in the deboosting mode thougha quantity of regenerative energy for the power circuit for a battery isthe same as that in the case of FIG. 15. As a result, the capacity ofthe capacitor can be reduced, and hence the promotion of reduction ofthe cost becomes possible.

In the power circuit for a battery of the present invention, since whenthe input electric power to the battery becomes equal to or less thanthe allowable input electric power, the electric power is transmittedfrom the capacitor to the battery through the DC/DC converter, acapacitor having a small capacity can be used.

Embodiment 5

FIGS. 21 and 22 are a flow chart of regenerative control of a powercircuit for a battery according to Embodiment 5 of the presentinvention. A configuration of the power circuit for a battery accordingto Embodiment 5 is the same as that of FIGS. 11 and 12. The flow chartof FIGS. 21 and 22 is partially different from that of FIGS. 18 and 19.However, the other constitution of FIGS. 21 and 22 is the same as thatof FIGS. 18 and 19. S301 to S304 and S307 to S322 are the same as S101to S104, S107 to S119, and S201 to S203 of FIGS. 18 and 19. Thus,different Steps are only S305 and S306.

In S305, the input current calculation means 13 calculates a batteryallowable input electric power PBMAX(W) on the basis of the batterytemperature TB(° C.) and the SOC of the battery. Then, the batteryallowable input electric power PBMAX is multiplied by a coefficient m(e.g., m is 0.5) used to obtain a predetermined value to obtain abattery allowable input electric power PBMAX(m) as a predeterminedvalue. Moreover, the input current calculation means 13 obtains abattery allowable input current IBMAX(m) from the battery allowableinput electric power PBMAX(m) and the battery terminal voltage VB.Similarly to the battery allowable input electric power PBMAX ofEmbodiment 3, the battery allowable input electric current PBMAX(m) isused in the processing in and after S307.

In addition, in S306, the input current I is compared with the batteryallowable input electric current IBMAX(m). If it is judged in S306 thatthe input current I is larger than the battery allowable input electriccurrent IBMAX(m), then the operation proceeds to S307. On the otherhand, if it is judged in S306 that the input current I is equal to orsmaller than the battery allowable input electric current IBMAX(m), thenthe operation proceeds to S320.

In such a power circuit for a battery, since the input electric power isadjusted to a predetermined value smaller than the battery allowableinput electric power, there is room in the input electric power. Sinceeven if an instantaneous regenerative electric power is inputted, thereis room in the input electric power, even when a large electric power isinstantaneously inputted, the charging can be carried out. In addition,since the predetermined value is adjusted to an electric power withwhich the degradation of the life can be kept less, the degradation ofthe first energy storage source is kept less.

Note that while in Embodiment 5, 0.5 is set as the coefficient used toobtain the predetermined value, the same effects can be obtained as longas the coefficient falls within the range of 0.3 to 0.8.

Embodiment 6

FIGS. 23 and 24 are a flow chart of regenerative control of a powercircuit for a battery according to Embodiment 6 of the presentinvention. A configuration of the power circuit for a battery accordingto Embodiment 6 is the same as that of FIGS. 11 and 12. The flow chartof FIGS. 23 and 24 is the same as that of FIGS. 18 and 19 except thatnew steps are added to the flow chart of FIGS. 18 and 19.

In the flow chart of FIGS. 18 and 19, when the vehicle speed Sv is zero,or the braking force PF is zero, the regenerative control is completed.On the other hand, in the flow chart of FIGS. 23 and 24, when in S101,the vehicle speed Sv is zero, or in S102, the braking force PF is zero,the operation proceeds to S401. In S401, the SOC(%) of the battery isobtained on the basis of the battery terminal voltage VB(V). In S402, itis judged whether or not the SOC of the battery is equal to or smallerthan a preset threshold of 10%. If it is judged in S402 that the SOC ofthe battery is equal to or smaller than the preset threshold of 10%,then the operation proceeds to S403. On the other hand, if it is judgedin S402 that the SOC of the battery exceeds the preset threshold of 10%,then the regenerative control is completed. In S403, the DC/DC converteris driven to shift the electric power from the capacitor to the batteryin order to charge the battery with electricity (the conversion of theelectric power by the DC/DC converter in this direction is referred toas a deboosting DC/DC converter operation).

In such a power circuit for a battery, when the SOC of the batteryreaches the vicinity of a preset threshold, i.e., a lower limit value ofthe allowable SOC, the electric power is shifted from the capacitor tothe battery in order to charge the battery with electricity. Hence, itis prevented that the SOC of the battery is remarkably reduced and underthis condition, the over-discharge state continues. Thus the life of thebattery is lengthened.

Note that while in Embodiment 6, when the SOC becomes equal to orsmaller than the threshold of 10%, the electric power is shifted fromthe capacitor to the battery in order to charge the battery withelectricity, even when a value in the range of about 5 to about 20% isset as the threshold, the same effects can be obtained.

INDUSTRIAL APPLICABILITY

As set forth hereinabove, the power circuit for a battery can beutilized for a hybrid automobile or the like having both an internalcombustion engine and an electric motor. Since the braking energy ismore regenerated with the electrical brake on be stored in the energystorage source, the rate of fuel consumption is improved.

1. A power circuit for a battery, comprising: a first energy storagesource; a second energy storage source connected in series with thefirst energy storage source, the second energy storage source having anallowable input current larger than an allowable input current of thefirst energy storage source; an electric power conversion circuit forconverting electric power between an electric motor which is connectedto an axle of a vehicle and the first energy storage source and thesecond energy storage source; a DC/DC converter for converting electricpower between the first energy storage source and the second energystorage source; and control means for controlling the DC/DC converter,wherein the control means includes DC/DC converter control means for,when regenerative electric power generated by the electric motor chargesthe first energy storage source and the second energy storage sourcethrough the electric power conversion circuit, controlling the DC/DCconverter so that an input current to the first energy storage sourcedoes not exceed the allowable input current of the first energy storagesource.
 2. The power circuit for a battery according to claim 1 furthercomprising: a vehicle speed sensor for detecting vehicle speed of avehicle; a braking command unit for issuing a braking command inaccordance with which braking is applied to the vehicle with a brakingforce; and a first voltmeter for detecting terminal voltage of the firstenergy storage source, wherein the control means comprises: inputcurrent calculation means for calculating the regenerative electricpower generated based on the vehicle speed detected and the brakingforce, and for calculating an input current with which the regenerativeelectric power charges the first energy storage source and the secondenergy storage source without an electric power shift in the DC/DCconverter; and allowable input current calculation means for obtaining astate-of-charge (SOC) of the first energy storage source, based on theterminal voltage of the first energy storage source, to calculate anallowable input current to the first energy storage source, based on theSOC; and when the regenerative electric power charges the first energystorage source and the second energy storage source through the electricpower conversion circuit, the DC/DC converter control means controls,when the input current obtained from the input current calculation meansis larger than the allowable input current obtained from the allowableinput current calculation means, the DC/DC converter so that theregenerative electric power is shifted from the first energy storagesource to the second energy storage source.
 3. The power circuit for abattery according to claim 2, wherein, when the regenerative electricpower charges the first energy storage source and the second energystorage source through the electric power conversion circuit, the DC/DCconverter control means controls, when the input current obtained fromthe input current calculation means is not larger than the allowableinput current obtained from the allowable input current calculationmeans, the DC/DC converter so that the regenerative electric power isshifted from the second energy storage source to the first energystorage source.
 4. The power circuit for a battery according to claim 1further comprising: a vehicle speed sensor for detecting vehicle speedof a vehicle; a braking command unit for issuing a braking command inaccordance with which braking is applied to the vehicle with a brakingforce; and a first voltmeter for detecting terminal voltage of the firstenergy storage source, wherein the control means comprises input currentcalculation means for calculating a regenerative electric powergenerated based on the vehicle speed detected and the braking force, andfor calculating an input current with which the regenerative electricpower charges the first energy storage source and the second energystorage source without an electric power shift in the DC/DC converter;and when the regenerative electric power charges the first energystorage source and the second energy storage source through the electricpower conversion circuit, the DC/DC converter control means controls,when the input current obtained from the input current calculation meansis larger than a value set in advance, the DC/DC converter so that theelectric power is shifted from the first energy storage source to thesecond energy storage source, and controls, when the input currentobtained from the input current calculation means does not exceed avalue set in advance, the DC/DC converter so that the electric power isshifted from the second energy storage source to the first energystorage source.
 5. The power circuit for a battery according to claim 2further comprising: a second voltmeter for detecting terminal voltage ofthe second energy storage source; and a brake mechanism connected to anaxle of the vehicle for mechanically braking rotation of the axle,wherein the control means comprises regeneration-enabling electric powercalculation means for, when the electric power is shifted from the firstenergy storage source to the second energy storage source through theDC/DC converter, calculating a regeneration-enabling electric power thatcan be regenerated in the first energy storage source and the secondenergy storage source based on the terminal voltage of the first energystorage source and the terminal voltage of the second energy storagesource, and mechanical brake control means for, when the regenerativeelectric power obtained from the input current calculation means islarger than the regeneration-enabling electric power, calculating amechanical brake operation quantity based on difference between theregenerative electric power and the regeneration-enabling electricpower, and for controlling the brake mechanism based on the mechanicalbrake operation quantity.
 6. The power circuit for a battery accordingto claim 5, wherein the second energy storage source includes acapacitor, and when the regenerative electric power charges the firstenergy storage source and the capacitor through the electric powerconversion circuit, the DC/DC converter control means controls the DC/DCconverter so that an applied voltage becomes no larger than an allowableapplied voltage of the capacitor.